(1) Field of the Invention
This invention relates to a glass ceramic dental crown and a method for manufacturing the same.
(2) Description of the Prior Art
The lost wax process has hitherto been known as the technique of fabricating a dental crown. To describe briefly, the process involves applying wax on a damaged tooth to prepare a wax pattern, investing the wax pattern with a mold material or an investing material, and hardening the investing material. The wax pattern is then thermally removed to provide a mold. A dental crown material is cast in the mold to fabricate the predetermined dental crown.
Hitherto, metallic materials including alloys have been used as a dental crown material. However, the metallic materials constitute a chemical battery in the oral cavity, and dissolve out metal ions within the oral cavity, tending to harmfully affect the human body. Moreover, the metallic material has a less attractive appearance, and, what is worse, frequently imparts an unpleasant feeling to the user due to different thermal conductivity between the natural tooth and the metallic dental crown.
Recently, a new artificial dental crown has been proposed which is prepared from crystallized glass of the calcium phosphate series. The crystallized glass of the calcium phosphate series has a composition resembling that of a natural tooth as described in Japanese Patent Disclosure No. 51-73019, and is regarded as adaptable as a dental crown material. At present, clinical research work is undertaken on the proposed new dental crown material. The proposed artificial dental crown is fabricated by adding a nucleating material to relatively low melting glass of the calcium phosphate series (for example, CaO-P.sub.2 O.sub.5, CaO-Al.sub.2 O.sub.3 -P.sub.2 O.sub.5), melting the mixture, molding the molten mass in the lost wax mold, and thermally treating the resultant transparent glass dental crown at a temperature approximating the crystal-precipitating level, thereby crystallizing the glass.
However, the conventional dental crown material prepared from the crystallized glass of the calcium phosphate series reacts with the investing material such as cristobalite during the heat treatment for crystallization. As a result, the surface of the fabricated dental crown is roughened to adversely affect its color and transparency. Further, the glass of the calcium phosphate series constituting the conventional dental crown has one-dimensional or two-dimensional structure, namely, chain-like structure, and consequently has a lower chemical durability than the silicate glass having a three-dimensional structure. Moreover, the conventional dental crown has further drawbacks that when subjected to thermal crystallization, the conventional dental crown presents difficulties in attaining a uniform and compact crystallized structure, and the crown is reduced in mechanical strength.